As a distributed electric power generator capable of effectively utilizing energy, a fuel cell cogeneration system (hereinafter simply referred to as “fuel cell system”) having high power generation efficiency and high overall efficiency has been attracting attention. The fuel cell system includes a fuel cell that is a component as a main body of a power generating portion, and additional components other than the fuel cell. Generally, the entire electric power generator including these components is collectively called a fuel cell system.
The fuel cell system includes a stack-type fuel cell (commonly known as a “stack”, and hereinafter simply referred to as a “fuel cell”), which is a component as the main body of the power generating portion. In the fuel cell, a predetermined number of unit cells (commonly known as a “cell”) are stacked. Examples of the fuel cell are a phosphoric acid fuel cell, a molten carbonate fuel cell, an alkali aqueous solution fuel cell, a polymer electrolyte fuel cell, and a solid electrolyte fuel cell. Among these fuel cells, operating temperatures of the phosphoric acid fuel cell and the polymer electrolyte fuel cell (abbreviated as PEFC) during an electric power generating operation are comparatively lower than those of the other fuel cells. Therefore, the phosphoric acid fuel cell or the polymer electrolyte fuel cell is typically used as the fuel cell constituting the fuel cell system. Especially, in the case of the polymer electrolyte fuel cell, an electrode catalyst does not deteriorate so much, and dispersion of polymer electrolytes does not occur as compared with the phosphoric acid fuel cell. Therefore, the polymer electrolyte fuel cell is especially preferably used in applications, such as mobile electronic devices and electric cars.
The polymer electrolyte fuel cell uses hydrogen in the electric power generating operation. However, generally, means for supplying hydrogen is not developed as an infrastructure. Therefore, in order to obtain predetermined electric power by the fuel cell system including the polymer electrolyte fuel cell, hydrogen needs to be generated at an installation location of the fuel cell system. On this account, in a conventional fuel cell system, a hydrogen generator is typically disposed with the fuel cell. The hydrogen generator uses water, and a hydrocarbon based material, such as a natural gas, a propane gas, naphtha, gasoline, and kerosene, or an alcohol based material, such as methanol, to generate a fuel gas containing hydrogen by, for example, a steam-reforming reaction. The polymer electrolyte fuel cell is supplied with hydrogen contained in the fuel gas and oxygen contained in the oxidizing gas, such as air, to output predetermined electric power.
In the electric power generating operation of the fuel cell system including the polymer electrolyte fuel cell, the polymer electrolyte fuel cell is supplied with the fuel gas and the oxidizing gas, each of which is humidified so as to have a predetermined dew point. Especially, in consideration of the life of the polymer electrolyte fuel cell, it is desirable that the polymer electrolyte fuel cell be supplied with the fuel gas and the oxidizing gas, each of which is humidified so as to have a dew point that is higher than a temperature of a predetermined region of the polymer electrolyte fuel cell to which region the fuel gas and the oxidizing gas are initially introduced. As above, since the polymer electrolyte fuel cell is supplied with the fuel gas and the oxidizing gas, each of which is humidified so as to have the predetermined dew point, or the fuel gas and the oxidizing gas, each of which is humidified so as to have the dew point that is higher than the temperature of the predetermined region, the electric power generating operation of the polymer electrolyte fuel cell is preferably carried out in the fuel cell system.
Generally, in the fuel cell system designed such that the oxidizing gas (hereinafter simply referred to as “off air”) discharged from the polymer electrolyte fuel cell has an adequate exhaust enthalpy, the oxidizing gas supplied to the polymer electrolyte fuel cell is easily and surely humidified up to a predetermined dew point by total enthalpy heat exchange with the off air in a total enthalpy heat exchanger.
Meanwhile, in the fuel cell system including the hydrogen generator, since a part of water added when causing the steam-reforming reaction to proceed remains in the fuel gas as steam, the fuel gas discharged from the hydrogen generator is being automatically humidified up to a certain degree. Therefore, as long as operating conditions of the hydrogen generator and operating conditions of the polymer electrolyte fuel cell adapt to each other, the fuel gas generated by the hydrogen generator can be directly supplied to the polymer electrolyte fuel cell without disposing a particular humidifier. However, in order to increase the efficiency of the steam-reforming reaction, it is desirable that the amount of water to be added be reduced in such a range that carbon deposition is not induced, and thereby latent heat of vaporization, of which a reaction system of the steam-reforming reaction is deprived, be reduced. In this case, since the amount of water to be added is reduced, the fuel gas may not be humidified up to a predetermined dew point in the hydrogen generator depending on the operating conditions of the polymer electrolyte fuel cell. Therefore, in the fuel cell system including the hydrogen generator, generally, a humidifier is separately disposed, which surely humidifies the fuel gas up to a predetermined dew point. The humidifier allows the fuel gas generated in the hydrogen generator to be surely humidified up to a predetermined dew point in the fuel cell system.
However, in the fuel cell system including the hydrogen generator, unlike the configuration of humidifying the oxidizing gas, it is very difficult to supply the fuel gas (hereinafter simply referred to as an “off gas”), discharged from the polymer electrolyte fuel cell, to the total enthalpy heat exchanger to humidify the fuel gas, generated in the hydrogen generator by the total enthalpy heat exchange, up to a predetermined dew point, even if the off gas has an adequate exhaust enthalpy. This is because when comparing the flow rates of the off gas and the off air discharged from the polymer electrolyte fuel cell in the electric power generating operation, the flow rate of the off gas is significantly lower than that of the off air.
More specifically, the operating temperature of a common polymer electrolyte fuel cell is about 60 to 80° C., and especially in the case of a cogeneration application, generally, the amount of cooling water to be supplied to the polymer electrolyte fuel cell is controlled such that a temperature difference between the cooling water supplied to the polymer electrolyte fuel cell and the cooling water discharged therefrom is about 10° C.
In this case, assuming that an oxygen utilization ratio (abbreviated as Uo) of the polymer electrolyte fuel cell is 50%, a molar flow rate of the off air discharged from the polymer electrolyte fuel cell is 90% of a molar flow rate of the oxidizing gas supplied thereto. That is, in a process of humidifying the oxidizing gas, the off air of the adequate temperature and flow rate is supplied to the total enthalpy heat exchanger. Therefore, in a case where the temperature of the off air discharged from the polymer electrolyte fuel cell is 70° C. for example, adjusting the dew point of the oxidizing gas to 60° C. by the total enthalpy heat exchange utilizing the off air of 70° C. is easily and surely achieved by using the total enthalpy heat exchanger designed appropriately.
However, as described above, it is very difficult to humidify the fuel gas up to a predetermined dew point by the total enthalpy heat exchange using the off gas. This is because since, generally, the fuel gas generated in the hydrogen generator is a mixture gas of hydrogen and carbon dioxide showing a ratio of about 8 to 2, and a fuel utilization ratio (abbreviated as Uf) of the polymer electrolyte fuel cell is about 80%, the molar flow rate of the off gas discharged from the polymer electrolyte fuel cell becomes about 40% of the molar flow rate of the fuel gas supplied thereto. That is, in the process of humidifying the fuel gas, an adequate flow rate of off gas (adequate amount of heat) is not supplied to the total enthalpy heat exchanger. Therefore, the fuel gas is not heated adequately. Therefore, even if the temperature of the off gas discharged from the polymer electrolyte fuel cell is 70° C., adjusting the dew point of the fuel gas to 60° C. by the total enthalpy heat exchange utilizing the off gas of 70° C. is very difficult even by using the total enthalpy heat exchanger designed appropriately.
Instead of the configuration of humidifying the fuel gas using the off gas as a heat source, a fuel cell system has been proposed, which humidifies the fuel gas using as the heat source the cooling water discharged from the fuel cell and increased in temperature (see Patent Document 1 for example).
In this conventional proposal, the cooling water discharged from the fuel cell and increased in temperature is utilized as the heat source when humidifying the fuel gas. Specifically, the cooling water discharged from the fuel cell and increased in temperature is supplied to the humidifier, and the off gas discharged from the fuel cell is supplied to the humidifier. Therefore, in the humidifier, the fuel gas generated in the hydrogen generator is adequately heated, and the off gas supplied to the humidifier is adequately heated. Thus, the fuel gas is humidified adequately.
Hereinafter, the configuration of a fuel cell system which humidifies the fuel gas using as the heat source the cooling water discharged from the fuel cell and increased in temperature will be outlined.
FIG. 9 is a block diagram schematically showing a part of a typical configuration of a conventional stationary power generating fuel cell system which humidifies the fuel gas using as the heat source the cooling water discharged from the fuel cell and increased in temperature. In FIG. 9, each of solid lines having arrows denotes a connection state between components in the fuel cell system and a flow direction of the fuel gas, the oxidizing gas, primary cooling water, or secondary cooling water in the electric power generating operation.
As shown in FIG. 9, a conventional fuel cell system 500 includes: an oxidizing gas supplying and discharging system including a blower 101, a total enthalpy heat exchanger 102, and a condenser 103; a fuel gas supplying and discharging system including a hydrogen generator 104 having a heater 104a, a humidifier 105 having a beater 105a, and a condenser 106; and a fuel cell 107 which is supplied with the humidified oxidizing gas and fuel gas from the total enthalpy heat exchanger 102 of the oxidizing gas supplying and discharging system and the humidifier 105 of the fuel gas supplying and discharging system to generate electric power.
Moreover, as shown in FIG. 9, the conventional fuel cell system 500 includes: a primary cooling water supplying and discharging system including a cooling water tank 108, a pump 109, the heater 105a, and a heat exchanger 110 for controlling temperatures of the fuel cell 107 and the humidifier 105; and a secondary cooling water supplying and discharging system including a cooling water tank 111, a pump 112, the condenser 103, the condenser 106, the heat exchanger 110, and a heat radiator 113 for controlling temperatures of the off air, the off gas, and primary cooling water discharged from the total enthalpy heat exchanger 102 or the humidifier 105.
To be specific, the conventional fuel cell system 500 includes the fuel cell 107 that is the component as the main body of the power generating portion, and the oxidizing gas supplying and discharging system, the fuel gas supplying and discharging system, the primary cooling water supplying and discharging system, the secondary cooling water supplying and discharging system, and the like that are the additional components other than the fuel cell 107.
In accordance with the conventional fuel cell system 500, when humidifying the fuel gas generated in the hydrogen generator 104, the cooling water discharged from the fuel cell 107 and increased in temperature is supplied to the heater 105a of the humidifier 105. The heater 105a adequately heats the humidifier 105. Therefore, since the heater 105a adequately heats the humidifier 105 even when the flow rate of the off gas discharged from the fuel cell 107 is low, the fuel gas supplied from the hydrogen generator 104 to the humidifier 105 is adequately heated. Also, the off gas supplied from the fuel cell 107 to the humidifier 105 is adequately heated. Therefore, the fuel gas supplied from the hydrogen generator 104 to the humidifier 105 is adequately humidified. Note that condensed water collected by the condenser 103 and the condenser 106 is purified through a predetermined purifying step, and is used as the cooling water or the like.
As the other fuel cell system which humidifies the fuel gas using as the heat source the cooling water discharged from the fuel cell and increased in temperature, a fuel cell system is proposed, in which water is directly poured into the humidifier, the poured water is heated by the cooling water discharged from the fuel cell and increased in temperature, and thereby the fuel gas is humidified. Since the fuel gas generated in the hydrogen generator is adequately humidified in the humidifier in the fuel cell system, the electric power generating operation of the fuel cell is preferably carried out (see Patent Documents 2 and 3 for example).
Patent Document 1: Japanese Laid-Open Patent Application Publication 2002-216816
Patent Document 2: Japanese Patent Application HEI 6-118149
Patent Document 3: Japanese Laid-Open Patent Application Publication HEI 7-226222